Water-repellant surface



Nov. 21, 1967 R. H. DETTREI ET AL 3,354,022

'y wATER-REPELLANT SURFACE Filed March 31, 1964 v 5 Sheetsfsheet sMEMS/NG ROLLER COL@ /7//P United States Patent 3,354,022 WATER-REPELLANTSURFACE Robert Harold Dettre, Wilmington, Harold Leonard Jackson,Hociressin, and Rulon Edward Johnson, Jr., Wilmington, Del., assignorsto E. I. du Pont de Nemours and Company, Wilmington, Del., a corporationof Delaware Filed Mar. 31, 1964, Ser. No. 356,155 8 Claims. (Cl.161--123) ABSTRACT OF THE DISCLOSURE This invention relates to waterrepellent, solid surfaces and to a process for their preparation. Moreparticularly, this invention relates to solid surfaces havingconfigurations which endow the surfaces with improved water repellentproperties and a method for imparting these configurations to solidsurfaces.

Prior to this invention, major efforts in combatting the problem ofproducing water repellent materials centered around the application ofcoatings to substrates. A variety of water impervious coatings have beenused, but they have the inherent disadvantages characteristic 4ofcoatings in general, namely, gradual absorption of water by the coatingand contact angle hysteresis both of which reduce repellency. These andother problems have prevented the art from producing a durable, highlywater repellent surface.

Literature concerning the theoretical requirements for good waterrepellency has created certain misleading postulates. For example, it isgenerally believed that water repellency is greatest when the contactangle 0 is as large as possible. Contact angle 6 is defined as the angle(measured through the liquid) which a liquid makes with a solid'. Whileit is true that increases in the advancing contact angle (the largestexperimentally measured contact angle, 0a) and receding contact angle(the smallest experimentally measured contact angle, 0r) can improverepellency, in certain instances increases in both these angles resultin decreased depellency. Water repellency, which is defined in terms ofthe angle a (relative to the horizontal) to which a surface must betilted in order for a water drop to roll off, depends on the differencesin the cosines of the advancing and receding contact angles. Thisrelationship is given by:

` (1) Sin a=KW (cos r-cos 0,) where K'=a constant for a given volume ofwater W=width of drop Therefore, as mentioned above, repellency can bereduced if the increases in 0r and 0a are such that the difference intheir cosines increases.

. The existing literature also gives the impression that formation of acomposite surface, i.e., a surface composed of high portions and lowportions which when contacted with water will create air-liquid andsolid-liquid interfaces between the water drop and the repellentsurface, will give improved repellency regardless of the air content ofthe composite surface. The air content of a composite surface isdetermined by taking an imaginary plane parallel to the surface passingthrough the tops of the high portions of the surface and measuring atthis plane the percentage of the total surface area which is air.

This hypothesis found in the prior art is partially based on the premisethat an increase in contact angle alone is sufficient to improverepellency. However, composite surfaces can be obtained which are lessrepellent than their smooth counterparts. FIGURE l illustratesgraphically the effect of the air content of a surface on the advancingcontact angle 9a and the receding contact angle 0!- for a representativesolid surface of this invention having a constant chemical compositionand an intrinsic advancing water contact angle (0a) of 110 and anintrinsic receding water contact angle (0r) of 92. The intrinsic contactangle (for 0 and 0,) is defined as the contact angle exhibited by agiven solid surface when smooth. By a smooth surface is meant a surfacehaving high portions not greater thanabout 0.5 micron in height. Whethera composite surface will be more or less repellent than thecorresponding smooth surface depends on the air content of the compositesurface. Actually, there is a reduction in repellency for compositesurfaces having low percentages of air in the surface when compared totheir smooth solid counterparts. This is due to the fact that theadvancing contact angle 0 increases more rapidly than does the recedingangle 0r when the air content of the surface is increased within certainlimits, i.e., from about 0 to about 60% as shown in FIGURE 1. Thisincrease in air content results in an increase in the difference betweenthe contact angles. Referring to Equation 1, this increase in differencecauses a concomitant increase in the tilt angle u.

FIGURE 2 further illustrates graphically the relationship between tiltangle a and air content of the fsurface. As shown in the graph, initialincreases in air content decrease water repellency until at a certainair content the l tilt angle equals the tilt angle for the smoothsurface.

Beyond this critical air content tilt angles decrease drastically. Thecritical air content is about 60% or morerfor most solid surfaces.Surfaces having low tilt angles when contacted with water dropletsexhibit characteristics similar to mercury droplets on glass.

The manufactures, process and theory of this invention will be betterappreciated by .reference to the drawing wherein several embodiments ofthe invention are illusi trated.

FIGURE 1, discussed above, is a graph summarizing the effect ofincreasing aircontent on 0a and 0r for a given solid surface. Y y

FIGURE 2, discussed above, is a graph summarizing the variations in tiltangle a with increasing air content for a given solid surface.

FIGURE 3 is a plan view in partial section of one embodiment of thepresent invention wherein b is the distance between the centers ofadjacent high portions,

' c is the distance between the outer extremities of adjacent highportions and d is the diameter of a high portion.

FIGURE 4 is a cross-sectional view of the embodiment represented inFIGURE 3 taken along line 2 2 wherein h is the height of a given highportion.

FIGURE 5 is a plan view in partial section of another specificembodiment of the present invention wherein b, c and d have the samemeaning as in FIGURE 3.

FIGURE 6 is a cross-sectional view of the embodiment represented inFIGURE taken along line 4--4 wherein h has the same meaning as in FIGURE4.

FIGURE 7 is a photomicrographic plan view of a representativenon-uniform composite surface of this invention wherein the dark areasindicate low portions and the light areas indicate high portions. Theangle of illumination of the original photograph was to the plane ofthe' surface.

FIGURE 8 is a diagrammatic side elevation view of apparatus forpreparing surfaces having the configurations dis-closed in thisapplication.

It is an object of this invention to provide a permanent,

solid, water repellent surface. Another object is to provide a methodfor the production of solid water repellent surfaces. A further objectis to provide intermediate means utilizable in processes for theproduction of these surfaces. These and other objects will becomereadily apparent from the following description and claims.

These objects are accomplished in accordance with lthe present inventionwhereby a surface is produced having configurations which impart agreater degree of water repellency to the surface than the correspondingunmodified solid surface.

The tendency for a solid surface to repel water is dependent uponcontact angle hysteresis, Le., the difference between advancing (0a) andreceding (0,.) contact angles of a water droplet on the solid surface.Where the difference in the above angles is great, there is a tendencyfor the water droplet to remain on the solid surface, i.e., the surfacedoes not repel water. However, when hysteresis is reduced by lesseningthe difference between 0a and c1I water droplets are repelled from thesolid surface and roll off the surface. When solid surfaces consistingof compositionsv having a high intrinsic contact angle are suicientlymodified to produce a composite surface having an air content greaterthan the critical air content, low contact angle hysteresis is observedand high repellency is obtained.

This can be theoretically explained by considering the thermodynamicproperties of a water drop on a composite surface. Qualitatively, theseconsiderations show that when a drop moves over a composite surface theadvancing portion of its periphery rnoves rapidly over the solid areasand comes to rest at solid-air boundaries or edges. At these points thecontact angle 0 increases to very high values, approaching 180 Thisbehavior reinforces the tendency of the drop, due to its surfacetension, to minimize its surface area. The receding portion of the dropperiphery tends to remain on solid regions and the contact angle 0r atthesey points approaches the receding contact angle for water on thesmooth solid. The extent to which this takes place depends on how muchhigh solid area is present in the composite surface. Opposing thetendency to remain on solid regions is the previously mentioned tendencyto minimize surface area. Therefore, as the air content of the compositesurface increases and less solid is present to restrain the periphery,the surface tension of the water predominates and the receding contactangle' increases to high values, approach ing those of the advancingportions of the drop.

Specifically, the present invention is directed to a solid surfacehaving an intrinsic advancing water contact angle 0a of greater than 90and a receding water contact angle 0r of at least 75, Said SOlid surfacecomprising high portions and low portions, said low portions being atleast 60% of the total surface area of the solid surface (i.e., an aircontent of at least 60% air).and said high portions having an averagedistance between adjacent high portions of not greater than 1000microns, the average height of the high portions above the low portions.being at least .5 times the averagedistance between adjacent highportions, said high portions having con gurations which produce a lowertilt angle on said solid surface than exhibited on the correspondingsmooth surface.

One embodiment of the present invention involves a solid surfacecomprising an @ordered arrangement of projections (high portions)consisting of non-contiguous vertically oriented columns of solid risingfrom the low surface, the maximum distance between the columns beinggiven by where Cm is distance in centimeters, 0a is the intrinsicadvancing water contact angle and f is air content expressed as afraction (i.e., the fraction of the total surface area that consists oflow portions), the height of the columns being at least .5 times thedistance between adjacent columns, said surface having an air content ofat least air (low portions) and the advancing intrinsic water contactangle being greater than 90 and the receding intrinsic water contactangle being at least Another embodiment of the present inventioninvolves a solid surface comprising high portions having therein anordered arrangement of depressions (low portions), said high portionsconsisting of interconnected vertical walls of solid which enclosechambers of air, the maximum distance between the walls being given byCm=0.587 cos 0a where C,m is the distance in centimeters, Qa is theintrinsic advancing water contact angle and the depth of the depressionsbeing at least .5 times the distance between the walls, said surfacehaving an air content of at least 60% air (depressions), the solidsurface having an intrinsic advancing water contact angle of greaterthan and an intrinsic receding water contact angle of at least 75.

A third embodiment involves solids having rough surfaces comprisingnon-uniform arrangements of high and low portions which on an averagefall within the necessary height, distance and air content requirementsgeneric to this invention.

Preferred embodiments of the present invention involve solid surfaceshaving intrinsic advancing water contact angles of greater than andintrinsic receding Water Contact angles of greater than 85 with aircontents of 75% air or above and heights of high portions above lowportions about equal to the average distances between adjacent highportions.

In systems comprising ordered arrangements of high portions(projections) the preferred distance between adjacent high portions isabout 250 microns or below. On surfaces having an ordered arrangement ofdepressions surrounded by interconnected vertical walls the preferreddistance between adjacent walls is 600 microns or below.

The water repellent surfaces of this invention may be made from anysolid surface having an intrinsic advancing water contact angle 0a ofgreater than 90 and `a receding Water contact angle (if of at least 75.It is necessary that the surfaces have high intrinsic advancing contactangles to prevent liquid penetration in the composite surface due to thehydrostatic pressure of the liquid drops. Receding contact angles of atleast 75 are necessary to avoid having to increase the air content ofthe surface to impractically high values to overcome the attractiveforces between the drops and the high solid portions.

Solids found to have the necessary intrinsic contact angles are forexample: alkyl polysiloxanes; polyfluoroalkyl polysiloxanes; waxeshaving a softening temperature greater than 40 C. which are essentiallyhalogen free and water insoluble; and polymers comprising polymerizableor copolymerizable ethylenic compounds which will form polymers havingthe following surface constitutions:

as described by Shafrin and Zisman, l. Phys. Chem. 64, 519, 521 (1960).

Speciiic examples of operable ethylenic compounds which fall within thedefinition of the invention are as follows:

where n is from 3 to 13.

Examples of polymers which arev not satisfactory are the homo-polymersof: CF2=CC12, CHZZCHOI,

CHFCClz, CHFCHCN The waxes which are operable in this invention are:vNatural waxes such as beeswax, candelilla wax, carnauba Wax, espartowax; petroleum waxes such as paratiin waxes, micro-crystalline waxes;fossil or earth waxes such as montan wax, ozocerite, peat wax andsynthetic waxes such as Fischer-Tropsch waxes and hydrogenated waxes.

While most solid surfaces exhibiting the required intrinsic contactangle show improved repellency over thesmooth unmodified surface whenthe air content is above about 60% and the other conditions mentionedabove are met certain surfaces (e.g., polyethylene) do not show improvedrepellency until the air content of the surface is about 75% to 80% air.

The expressions listed above for the distance between adjacent highportionsare derived from a consideration of the water penetrationpressure on the surfaces as follows: When a cylindrical capillary ismade of a material which when smooth has an advancing water contactangle, 0a, greater than 90, a pressure, P, is required in order 6. toforce the water into the capillary. The magnitude of the pressure isgiven by 2 P=47 cos 6a where Fy is the surface tension of water and c isthe diameter of the capillary. Using the value for the surface tensionof water at 25 C. and expressing P in centimeters of water, Equation 2becomes cos 6a cos 0, c

where 0r is the receding contact angle. if Hr is less than the waterwill show no tendency to come out of the capillary. For most solids withnon-zero Contact angles, @a and 0, are not equal (0r being less than0a). In this situation the angles 0a and 0r are not to be confused withthe angles actually observed forl water drops on the embossed surface.The present angles 9a and 0r refer to the water angles on the solidregions of the embossed surface. In some instances surfaces can beprepared where 6:0r but this requires special procedures to obtainsmooth surfaces and elaborate purication methods to obtain homogeneoussurfaces; It is probably impossible to obtain homogeneous polymersurfaces; even adsorbed monolayers are, to some extent, heterogeneous.Many materials possess surfaces when smooth which have 0g greater than90 and 0r less than 90. It is therefore necessary to emboss the surfacesof these materials in order to have high repellency. For a given surface(with a 9a 90) the penetration pressure is determined by the distance,c,- in Equation 3. The maximum distance that could be tolerated would beone which prevents water drops from penetrating under their ownhydrostatic pressure. The maximum 'height that a water drop can attainin the presence of gravity is about 0.5 cm. The maximum distance, Cm (incm.), is then given by i (5) Cm=0.587 cos 0,L

edv cos 0a where d is the diameter of the cylinders and b is thecenter-to-center distance between cylinders. Expressed in terms of f(the fraction 'of the total surface area that is air) and the distance,c, between the outer walls of the cylinders (c=b-d) and using the valuefor the surface tension of water at 25 C., Equation 6 becomes P=[\/2.721 1-f) -rraau-fo'g) @OS 0 Where P is expressed in centimeters ofwater. ImposingV the condition that a water drop should not penetrateinto the surface under its own hydrostatic pressure gives for themaximum distance between cylinder walls,

In a system of random dispersed high portions (rough surfaces) it isdifficult to give exact mathematical definitions of distance ranges.However, prepared surfaces having average distances between adjacenthigh portions falling within the generic requirements of this inventionprovide improved water repellency. Preferred non-uniform surfaces havingdistances of from about 20 microns to about 400 microns have been foundto impart superior water repellency to surfaces provided the largestaverage spacing between adjacent high portions is below about 300microns.

The high portions of the composite surfaces of this invention must havecertain configurations in order to provide the proper air content whileat the same time avoiding penetration of the liquid into the lowportions of the surface. Optimum results are achieved when the sides ofthe walls or projections are perpendicular to the plane of the surface,however, it is not necessary that the sides of the projections or wallsbe absolutely vertical or perpendicular to the surface. It is sufficientif somewhere on all sides of a given high portion there is a point atwhich a line drawn tangent to the side will make an angle between thetangent and the horizontal, measured through the high portion, of about80 or more.

Methods of preparing water repellent surfaces The present invention isalso directed to a process for the preparation of water repellent solidsurfaces which comprises contacting a solid surface having an intrinsicadvancing water conta-ct angle of greater than 90 and an instrinsicreceding contact angle of at least 75 with means for softening saidsolid surface, contacting the solid surface with a die capable ofproducing a positive configuration comprising high portions and lowportions on the solid surface wherein the height of the high portionsand the distance between adjacent high portions will conform to therequirements previously stated and removing said die from contact withthe solid surface.

Alternatively, the softening step may be dispensed with provided the dieform or mold is of sufficient mechanical strength to withstand pressuresnecessary to impart the desired configurations to the surface materialsof this invention without damage to the die or loss of detail in theresulting surface configuration.

Die forms or molds utilizable in the practice of this invention includecommercially available screens or mesh plates, photosen-sitive polymerswhich after exposure to actinic light are treated to produce thenegative of the desired surface configuration, metallic dies produced bycontacting the metal with photosensitive polymers having the positiveconfiguration desired to be reproduced on the solid surface and massesof hollow fibers rigidly held together by suitable means. These holiowfiber die forms or molds are aligned together in parallel manner inbundles and cut or sliced approximately perpendicular to their length tocreate a die surface of their cross-section. The outer diameter of thesehollow fibers must be on an average in the range of from about l toabout 450 microns and the inner diameter must be on an average less thanabout .6 times the outer diameter. The length of hollow fibers used inthe die must be at least .5 times the difference between the inner andouter diameters of the hollow fibers.

I. Surfaces consisting of projections A novel and convenient method forpreparing a surface consisting of vertical projections has now beenprovided by utilizing a mass of hollow fibers as a die form or mold. Thehollow fibers may be obtained as described in U.S. Patent 2,999,296 andFrench Patent 990,726. The outer diameter of the fibers used measurewithin the range of 10 to 450 microns and are 1.2 to 3.0 times the innerdiameter. The fibers are compacted together and cut or slicedapproximately perpendicular to their length exposing the cross-section.The truncated ends are then used as a mold or die. The surface to betreated is softened slightly, by heating or other suitable means (e.g.,solvents) to facilitate impression of the surface by the mold. Thesurface hardens as it cools, whereby high portions (projections)resembling vertical columns corresponding to the hollow cores of thefibers are permanently developed on the surface. The hollow fibers maybe assembled into bundles of various shapes and sizes or may be attachedto a roller having the truncated ends of the fibers as the rollersurface. A large quantity of the object having the surface to be treatedmay be conveyed in such a manner under the roller so that the pattern isstamped on the surface.

II. Surfaces consisting of depressions One method of producing a patternof depressions on a surface is now provided by first preparing a -gridconforming to the required surface dimensions by drawing the pattern ona sheet of paper which transmits actinic light. The pattern so drawn isa model of the pattern desired on a large scale. By reducingphotographically, the pattern is obtained in the small scale desired. Ahalftone screen, such as is used in photography, may also be used inplace of the paper pattern. A relief image of the pattern is made onphotosensitive plastic by placing the paper pattern on halftone screenover the plastic and exposing to actinic light, as described in U.S.Patent 2,760,- 863. There is formed on the plastic a relief image of thepattern. This plastic grid is now used as a die and impressed ontoanother surface capable in itself of being used as an embossing die,eig., Woods metal. This die, bearing a reverse pattern of the grid, isimpressed on the surface to be made water repellent in the same manneras described above for the hollow fiber surface. The lines of theoriginal paper pattern thus become the walls of the depressions. Theembossing dies may also be made of any metal or non-metal which can bechemically or mechanically modified to give the desired surface patternwith the desired spacings and dimensions, providing the material is ofsufficient strength to permit its use as a die. Alternatively, aphotosensitive glass available commercially, such as described in GlassEngineering Handbook, E. B. Shund, McGraw-Hill, New York, 2nd ed., 1958,p. 363, may be used in the preparation of such a surface. By followingthe lmethod described in the above reference for preparing patternedglass, a surface having an array of depressions may be produceddirectly.

However, glass in itself does not possess the requisite advancing andreceding water contact angles. It must therefore be treated with amaterial having the necessary intrinsic advancing and receding watercontact angles to achieve the desired water repellency by a physicalcoating process. Or, for example, monoor multilayers of a substancehaving the required Contact angles may be chemisorbed on the glass toform a strongly adherent coating. The coating acquires the configurationof the glass substrate thereby forming a highly water repellent surface.

The following examples are given to illustrate various specificembodiments of this invention and do not limit the invention in any way.

Contact angles were measured directly on profiles of sessile drops usingthe general method described in Surface Chemistry by I. J. Bikerman,Academic Press, Inc., New York (1958), 2nd ed., p. 343. In theseexamples contact angles were measured on profiles of sessile drops usinga microscope fitted with a goniometer eyepiece; magnification was about20X. Error in angle measurements by this method is estimated to be aboutif. Drop addition and size changes were made using a hypodermic syringe.Advancing angles were measured after the drop size was increased and theperiphery advanced over the surface; receding angles Were measured afterthe drop size was reduced and the periphery receded. Unless otherwisestated, readings were taken with 30 seconds of drop formation and/orsize change, and drop volumes were between 0.03 and 0.10 ml. Recedingangles were measured on drops that had been in contact with the surfacefor some time (several minutes) before being receded and on fresh dropswhich were placed on the surface and immediately (several seconds)reduced in size. There was little difference between these two methodsas long as the drop size was kept below 0.1 ml. Each contact angle valueis the average of at least 8 measurements. All measurements were made at25 il C. and 50% relative humidity.

In these examples the air content of the surface was determined byphotomicrographs (plan view) of the surface. The photomicrographs aretaken with the plane formed by the tops of the high portions of thesurface in focus. The fraction of the total area of the photomicrographsthat is air is taken as the air content of the surface. Themagnification is chosen such that each photomicrograph includes enoughof the area of the surface to give a reliable and reproducible value forsurface porosity (air content).

The method used to determine tilt angles (a) in these examples isessentially that described in I. Colloid. Sci., vol. 17, p. 309 (1962),by C. G. L. Furmidge. The surface to be studied was placed on a hingedbrass plate. A drop of water of the required volume was placed on theexperimental surface using a hypodermic syringe. The hinged plate wasimmediately `rotated until' the 'drop started to roll off the surface.The angle to which the plate was rotated was then measured .to thenearest degree lwith a protractor. With only slight differences the tiltanglennecessary to cause the drop to commence rolling is equal to theangle necessary to keep the drop rollmg.

EXAMPLE 1 An embossing die is made in the following manner: A bundle ofhollow fibers of a copolymer of tetraiiuoroethylene andhexauoropropylene is compressed, using a hose clamp, to give aclose-packed arrangement of the fibers. The hollow fibers have an insidediameter of 250 microns and an outside diameter of 430 microns. Theclamped Ibundle is then cut with a microtome knife so that the surfacethereby formed is iiush with one end of the hose clamp. This surface(1.5 cm. in diameter), which consists of the truncated ends of thehollow fibers, is used as an embossing die.

The die described above is pressed (by hand) against the warm surface ofa piece (2 cm. x 2 cm. x 1 cm.) of paraffin wax. The surface of the Waxis previously heated to 35 to 40 C. (to a depth of about 1 mm.) in orderto soften it and thereby facilitate fiow into the holes of the die.There is negligible adhesion of the paralin wax to the hollow fiber die.The embossed surface consists of an array of cylindrical projectionsoriented at right angles to the original wax surface. There isnegligible penetration of wax into the spaces between the fibers of thedie because these spaces are sufficiently small, the applied pressureduring embossing is low enough and the flow properties of the wax at 35to 40 C. are poor enough to prevent penetration.

The cylindrical projections are about 250 microns in diameter and 150 to200 microns high. The distance between centers of adjacent projectionsis about 430 microns and the area of the surface, measured at the planeformed by the tops of the projections, is approximately 70% air. Theadvancing contact angle for a water drop has increased from 113 on theoriginal surface to 144 on the embossed surface and the receding anglehas increased from 100 to 129. The tilt angle necessary for a 0.05 ml.water drop to roll olf the surface has decreased-from to 9.

A lower tilt angle for one surface than for another means that if bothsurfaces are vertical the surface with the lower tilt angle will permitroll-off of smaller water drops. This is equivalent to saying thatliquid 'retention will be lower on the surface with the lower tiltangle.

In actual practice the hollow fiber dies can be many times larger inarea then the one described above and the embossing process can becarried out using conventional presses. For example, the fibers can bemounted in closepacked array on any rigid backing such as a metal orhard plastic plate, the dimensions of the plate being determined by thesize of the surface to be embossed or by the size of the embossingpress. A plate of several square feet in area is possible. The closepacking can be achieved and maintained by exerting lateral pressure atthe periphery of the plate or by sealing the fiber ends to the plateusing a suitable adhesive. l

Alternatively, for applications where continuous embossing of largeareas is desirable, the fibers can be mounted on a roller of severalfeet in length and of sufficient diameter so that close packing of thefibers is possible at the outer surface of the die.

The parafiin wax in this example need not be a solid piece of the waxalone. It can just as well be a thin coating of wax (as thin as 0.1 mm.)on another surface, such as a sheet of glass or metal. I'

EXAMPLE 2 An embossing die is made in the manner described in Example 1using hollow fibers of polyethylene. These fibers have an insidediameter of 80 to 95 microns and an outside diameter of 220 microns.This die (1.5 cm. in diameter) is used to emboss the parafiin waxsurface of Example 1 using the method described in Example 1.

The resulting -cylindrical projections of the embossed surface are 80 to95 microns in diameter and 200 to 300V microns high. The distancebetween centers of adjacent projections is 240 to 300 microns and thearea of the surface, measured at the plane formed by the tops of theprojections, is approximately 87% air. The advancing contact angle for adrop of Water has increased from 113 to 156 and the receding angle hasincreased from 100 to 148. The tilt angle necessary for a 0.05 ml. waterdrop to roll off the surface has decreased from 15 to 4.

As with the hollow fiber dies of Example 1, it is also possible tofabricate dies which are much larger than the one used in this example.

EXAMPLE 3 The hollow ber die of Example 1 is pressed (by hand) againstthe surface of a piece of polypropylene film (2 cm. x 2 cm. x 0.05 cm.)having a melting point of about 165 C. and a melt index of about 0.5.The film is previously heated to 150 to 155 C. to soften it and therebyfacilitate flow into the holes of the die. The die is also heated (to to120 C.). The embos-sed surface consists of an array of cylindricalprojections 250 microns in diameter and 500 to 6.00 microns high. Thereis negligible penetration of polypropylene between the fibers of thedie. The average distance between centers of adjacent projections isabout 450 microns and the area of the surface, measured at the planeformed by the tops of the projections, is approximately 75% air. The ad`vancing contact angle for a water drop has increased from on theoriginal fiat surface to 139 on the embossed surface and the recedingContact angle has increased from 85 to 115. The tilt angle necessary fora 0.10 ml. water drop to roll off the surface has decreased from 21 to16.

The larger dies described in Example 1, particularly the roller type,can be used to emboss large sheets of polypropylene to produce thesurface described in this ex. ample.

1 1 EXAMPLE 4 The hollow fiber die of Example 1 is pressed (by hand)against the surface of a piece of polyethylene film (2 cm. x 2 cm. x 0.1cm.) having a melting point of about 110 C. and a melt index of 1.8. Thefilm is previously heated to 102 to 105 C. to soften it and therebyfacilitate ow into the holes of the die. The die is also heated (to 95to 100 C.). The embossed surface consists of an array of cylindricalprojections 250 microns in diameter and 450 to 500 microns high. Thereis negligible penetration of polyethylene between the fibers of the die.The average distance between centers of adjacent projections is about450 microns and the area of the surface, measured at the plane formed bythe tops of the projections, is about 75% air. The advancing contactangle for a water drop has increased from 98 on the original at surfaceto 140 on the embossed surface and the receding contact angle hasincreased from 76 to 105 The tilt angle necessary for an 0.05 ml. waterdrop to roll off the surface has decreased from 30 to 22.

EXAMPLE 5 A relief image of the configuration desired is preparedaccording to the method described in U.S. Patent 2,760,863, whereby thesurface of a photosensitive polymer (1 mm. thick coating of the polymeron a 2.5 mm. thick sheet of aluminum) is irradiated with actinic lightwhich has first passed through a photographic transparency on whichthere is an ordered array of opaque squares separated by ne spacingsthat transmit the actinic light. The exposed regions of thephotosensitive polymer undergo a chemical reaction which makes them moreresistant than the unexposed regions to attack by caustic solution.After a thorough washing with caustic solution the resulting polymersurface consists of a network of interconnecting walls (intersecting atright angles) enclosing square-shaped depressions. The walls are 27microns thick and 130 microns high. The distance between walls(centerto-center) is 195 microns. The network is approximately 1 cm.square in area.

An impression of this network is obtained by covering the surface withmolten (80 to 100 C.) Woods metal (a 0.5 cm. thick layer of the liquidmetal covers the surface), applying a vacuum (0.1 mm. of mercury) toremove air from between the Woods metal and the polymer surface, andthen restoring atmospheric pressure in order to force the liquid metalinto the depressions of the polymer surface. rIhe solidified metalimpression is used to emboss the paraiin wax surface of Example 1. Thesame embossing procedure is used as in Example 1.

Alternatively the negative form of the desired surface configuration canbe produced on the photosensitive polymer. However, a caustic Washingprocedure is necessary to -completely flush out the narrow (27 microns)channels which result if the negative form is made. Therefore, theprocedure utilizing the Woods metal impression is preferred in order toobtain the negative form for use in embossing other surfaces.

The embossed wax surface is a replica of the original photopolymersurface. However, some loss in surface detail has resulted from theabove procedure of making an impression of an impression. Thecenter-to-center dis tance between walls is 190` microns; the walls are37 microns thick and 130 microns high. The area of the surface, measuredat the plane formed by the tops of the walls, is 65% air. The advancingcontact angle for a water drop has increased from 113 on the originalflat surface to 137 on the embossed surface and the receding angle hasincreased from 100 to 116. No decrease in tilt angle was obtained heresince this example represents the lower limit to the air content of aparafin wax surface.

EXAMPLE 6 An embossing die is produced in the manner described inExample 5 and a at parafn wax surface is embossed in the mannerdescribed in Example 1. The embossed wax surface consists ofinterconnecting walls 15 microns thick and 130 microns high. Thedistance between walls (centerto-center) is 195 microns and the area ofthe surface, measured at the plane formed by the tops of the walls, isabout air. The advancing contact angle for water has increased from 113on the original flat surface to 155 on the embossed surface and thereceding contact angle has increased from 100 to 145. rl`he tilt anglenecessary for a 0.015 ml. water drop to roll off the surface hasdecreased from 15 to 4.

In actual practice the embossing dies described in this example and inExample 5 can be many times larger and can be used in plate or rollerform. The Woods metal die described here would be applicable only tosurfaces of materials which have the desirable flow characteristics attemperatures below 65 C. For other materials, dies of higher meltingmetals or alloys must be used.

If suficient care is taken in washing technique, the negativephotopolymer surfaces can be fabricated and used directly as embossingdies.

EXAMPLE 7 A fluorocarbon wax dispersion in 1,1,2-trichloro-1,2,2triiiuoroethane, obtained by reacting methanol and tetraiiuoroethylenein the manner described in U.S. Patent 3,067,262 (the Wax having acrystalline melting point of 278 C., an approximate molecular weight of2000) and containing about 20% solids, is diluted with a solution of 50parts of trichloroiiuoromethane and 50 parts of dichlorodifluoromethaneto give a solids concentration of about 1%. This mixture is sprayed ontoa 1" x 3 glass microscope slide to give a coating which is 0.1 to 0.2mm. thick. All components of the above mixture except the wax arevolatile. The wax particles at the top of the coating are 5 to 80microns in diameter, the spacing between them is 20 to 160 microns andthe area of the surface in the plane formed by the top of the coating,is approximately 88 to 90% air. The advancing contact angle for a waterdrop on this surface is 159 and the receding angle is 156; thecorresponding angles for a smooth, flat surface of the wax are 111 and95 re spectively. The tilt angle necessary for a 0.05 ml. water `drop toroll off the surface is 18 for the smooth wax and about 1 for thesprayed wax.

EXAMPLE 8 A mixture containing 90 parts of a 12.5% solution of paraiiinwax in n-hexane and 10 parts of glass beads of a diameter in the range 3to 12 microns, is heated to 40 to 50 C. and sprayed onto a 1" x 3 glassmicroscope slide to give a coating which is 0.1 to 0.2 mm. thick. Then-hexane rapidly evaporates from the surface leaving agglomerates ofwax-coated glass beads. The agglomerates at the top of the coating are15 to 250 microns in diameter, the spacing between them is 50 to 350microns and the area of the surface in the plane formed by the top ofthe coating, is approximately 88% air. The advancing contact angle for awater drop on this surface is 158 and the receding angle is 156; thecorresponding angles on a smooth, flat paran wax surface are 113 and 101respectively. The tilt angle necessary for a 0.05 ml. water drop to rolloff the surface is 15 for the smooth wax and about 1 for the sprayedwax-glass bead mixture.

In Examples 7 and 8 above, all parts are by weight.

EXAMPLE 9 A 400 line per inch nickel mesh plate, commercially availablefrom the Buckbee Mears Co., St. Paul, Minn. (6" x 6) with a square arrayof holes (28 to 29 microns square with a center-to-center distance of 60microns and a depth of 25 microns) is used as a die to emboss a lm (3cm. x 6 om. x 15 cm.) of a copolymer of tetrafluoroethylene andhexaiiuoropropylene. The plate iS 13 pressed against the film in aCarver laboratory press at 12,000 to 14,000 p.s.i. and 100 C. for 15minutes. The plate` is then peeled from the film; there is negligibleadhesion between plate and film. The embossed surface consists. of aregular 'l arrayof square-shaped projections oriented at right angles tothe original film surface..'l`hese projections are 28 to 29 microns on aside and 22 to 25 microns high. The centertocenter distance of adjacentprojections is 60 microns and the area of the surface, measured at theplane formed by thetops of the' projections is 75 to 78% air. Theadvancing vangle for a water drop has increased from 114V on theoriginal surface to 157H on the embossed surface and the receding anglehas 4increased from 97 to 134. Thetilt angle necessary for a 0.05 ml.drop to roll off the surface has decreased from'23 on the smooth surfaceto 12 on the embossed surface. v EXAMPLE 10 The nickel mesh plate ofExample 9 above is pressed against the surface of -a film ofpolypropylene (3 cm. x 6 cm. x 0.05 cm.) at 16,000 p.s.i.,and 140 C. for20 minutes. The plate is peeled from"the film; there is negligibleadhesion between plate and film. The embossed surface is asdescribed inExample 9 above. The advancing angle for a-water drop has increased from107 on the original surface to 158 on the embossed surface and thereceding angle has increased from, 83 to 110. The tilt anglenecessary-forv a 0.05 mlfdrop to roll off the surface has decreased from34 to 22.

EXAMPLE 1l A fiuorocarbon wax having a melting range of 95 to 135 C. andthe general formula where n varies from 3 to 9 but is predominately 7and 8, is dissolved in 1,1,2-trichloro-1,2,2-triuoroethane to the extentof about 5%. About 100 parts of the above solution and parts of glassbeads having diameters in the range of 3 to 12 microns are mixedtogether, heated to about 40 C. and sprayed onto a 1 by 3 glassmicroscope slide to give a coating which is 0.1 to 0.2 mm. thick. The1,1,2-trichloro-1,2,2-trifluoroethane rapidly evaporates from thesurface leaving agglomerates of waxcoated glass beads. The agglomeratesat the top of the coating are to 300 microns in diameter, the spacingbetween them is 80 to 300 microns and the area of the surface in theplane formed by the top of the coating, is approximately 82% air. Theadvancing contact angle for a water drop on this surface is 157 and thereceding angle is 154; the corresponding angles on a smooth, flatsurface of the wax are 128 and 114 respectively. The tilt anglenecessary for a 0.05 ml. water drop to roll off the surface is 13 forthe smooth wax and about 1 for the sprayed wax-glass `bead mixture. Inthe above example, all parts are by weight.

EXAMPLE 12 A continuous film of polyethylene (3 ft. wide, 150 micronsthick) having a melting point of 110 C. and a melt index of 1.8 ispassed beneath a suitable heater which heats the top half of the film to102 to 104 C. The lm is then passed between two rollers. The surface ofone roller is covered with the close-packed ends of hollow fibers of acopolymer of tetrauoroethylene and hexafluoropropylene. The hollowfibers have an inside diameter of 50 microns and an outside diameter of123 microns. The surface of the embossing roller is heated to 95 to 100C. just before it comes into Contact with the film as shown in FIGURE 8.This facilitates 4fiow of the polyethylene into the holes of the die.Before the film is separated from the embossing roller, a stream of coldair is directed -at the film in order to cool it to about C. Adhesion ofthe polyethylene to the copolymer is negligible and the film readilyseparates from the surface of the embossing roller.

The embossed surface consists of an array of cylindrical projections 50microns in diameter and 200 microns high. The average distance betweencenters of adjacent projections is 123 microns and the area of thesurface, measured at the plane-formed by the tops of the projections, isair. The film thickness, excluding the projections,v isnow microns. Theadvancing Contact angle for a water drop has increased from 98 on theoriginal fiat surface to on the embossed surface and the receding-anglehas increased from 76 to 120. The tilt angle necessary for a 0.05ml. water drop to roll olf the surface has decreased from 30 to 18".

Several of these 3-feet sheets, sealed together, can be used in showercurtains to make them more resistant to accumulation of undesirable filmsince much of this accumulation on ordinary curtains results fromwetting of the surface and subsequent evaporation of the water thereon.l y

The embossed surfaces prepared as described in the preceding examplesexhibit a lower tilt angle and thereby a vastly improved waterrepellency. The configuration of projections or depressions causes theadvancing and receding contact angles to increase so that the drop widtha'nd the difference in the cosines of the receding and advancing contactangles are decreased. The high water repellency is thus due to theintrinsic structure of the surface.

The novel water repellent surfaces of the invention are useful in theproduction of numerous articles which shed water easily, such asranwear, shower curtains, tenting material and plastic and ceramictiles. These surfaces may also constitute condenser surfaces of solarstills land serve as surfaces where ice formation is to be minimized.

While the composite surfaces of this invention have ben shown to haveutility in repelling water, they also sh'ow improved repcllency to otherliquids both water and oil based which exhibit intrinsic advancingcontact angles of greater than 90 and intrinsic receding contact anglesof at least 75. For example, liquids with surface tensions considerablylower than that lof water ('yf=72.8 dynes/cm. at 20 C.) will showincreased repellency on the spray-coated surface of Example 11. Thatcomposite surface has shown improved repellency over the corre sponfdingsmooth, flat surface to motor oils with surface tensions las low as 32dynes/cm. Examples of other liquids to which the surface of Example 11will show improved repellency include:

'y (dynes/cm. at 20 C.)

Composite surfaces of this invention have shown improved repellency toliquids with viscosities in the range of from 5,000 to 10,000centipoises and surface tensions greater than 40 Idynes/cm. and viscous,aqueous sugar solutions with viscosities of 3000 centipoises and surfacetensions close to that of water. Aqueous solutions of inorganic saltswill also be repelled more readily since their surface tensions rangefrom 72 to 85 dynes/cm.

As many apparently wi'dely different embodiments of this invention maybe made Without departing from the spirit and scope thereof, yit is tobe understood that this invention is not limited to the specificembodiments thereof.

What is claimed is:

1. A solid composition having projections extending from the surfacethereof, said projections having an average rdistance of not greaterthan 1000 microns between adjacent projections and having an averageheight of -at least .5 times the avenage distance, said projectionsspaced such that the air content of the surface of the composition is atleast 60% air, said projections shaped such thiat a line drawn tangentto the sides thereof makes an angle between the tangent land the planeof said surface of =at least 80 when measured through the projection,said surface 'of said projection having an intrinsic advancing Watercontact angle of greater than 90 and an intrinsic receding water contact:angle of aft least 75, said surface having a lower tilt angle for waterthan the corresponding smooth surface.

2. The composition vof claim 1 wherein the average distance betweenadjacent projections is not greater than 600 microns and the avenageheight of the projections is equal to the average distance betweenadjacent projections, wherein said |air content is at least 75% air, andwherein the intrinsic advancing Water Contact angle is greater than 100and the intrinsic receding water Contact angle is at least 85.

3. The composition of claim 1 wherein said composition is polyethylene@and wherein the ail content is at least 75 air.

4. The composition Iof claim 1 wherein said projection-s are11o-contiguous, vertically oriented celums, and wherein the miaximumdistance between columns is given by the equation {1339} COS wherein Cmis the distance in centimeters, 6EL is the intrinsic advancing watercontact angle and A is the ai-r content expressed las 'a fraction.

5. The composition of claim l-wh'ere'in said projections are orderedinterconnected vertical walls and wherein the maximum distance between|adjacent Walls is given by the equation 'Cm=0.587 cos 6a where Cm isthe distance in centimeters and 0a is `the intrinsic advancing Watercontent langle.

6. The composition of claim 1 wherein `said projections arenonuniformlly arranged, and wherein the average distance betweenadjacent projections is not greater than 300 microns. .f

7. The composition of claim 1 in the form of a film having saidprojections Ion one side thereof. v

8. The composition of claim 1 in the form of a sheet having saidprojections on one `side therof.

References Cited Bird Y 161- 116 MORRIS sUssMAN, Primary Examiner.

1. A SOLID COMPOSITION HAVING PROJECTIONS EXTENDING FROM THE SURFACETHEREOF, SAID PROJECTIONS HAVING AN AVERAGE DISTANCE OF NOT GREATER THAN100 MICRONS BETWEEN ADJACENT PROJECTIONS AND HAVING AN AVERAGE HEIGHT OFAT LEAST .5 TIMES THE AVERAGE DISTANCE, SAID PROJECTIONS SPACED SUCHTHAT THE AIR CONTENT OF THE SURFACE OF THE COMPOSITION IS AT LEAST 60%AIR, SAID PROJECTIONS SHAPED SUCH THAT A LINE DRAWN TANGENT TO THE SIDESTHEREOF MAKES AN ANGLE BETWEEN THE TANGENT AND THE PLANE OF SAID SURFACEOF AT LEAST 80* WHEN MEASURED THROUGH THE PROJECTION, SAID SURFACE OFSAID PROJECTION HAVING AN INTRINSIC ADVANCING WATER CONTACT ANGLE OFGREATER THAN 90* AND AN INTRINSIC RECEDING WATER CONTACT ANGLE OF ATLEAST 75*, SAID SURFACE HAVING A LOWER TILT ANGLE FOR WATER THAN THECORRESPONDING SMOOTH SURFACE.